The present invention relates to a chair backrest of the type defined in the preamble of the main claim.
The document WO-A-0193723 by the same Applicant describes a chair backrest comprising a support structure bearing an upper backrest member and a lower backrest member, oscillating in mutually independent fashion about respective mutually parallel transverse axes. The position of the lower backrest member is adjustable in the vertical direction and each of the two backrest elements is associated to respective elastic means which maintain the two backrest elements in a predetermined resting position and produce an elastic reaction force which tends to oppose the oscillation of the backrest members about the respective transverse axes.